P-type OFET with fluorinated channels

ABSTRACT

The present invention provides an organic field-effect transistor (OFET) and a method of fabricating the OFET. The OFET, configured to function as a p-type semiconductor, includes a substrate having a top surface and a semiconductor layer located over the top surface. The semiconductor layer comprises organic semiconductor molecules. Each of the organic semiconductor molecules includes a core having conjugated pi bonds, a fluorinated alkyl group, and an alkyl spacer group having a chain of two or more carbon atoms. One end of the chain is bonded to the fluorinated alkyl group and another end of the chain is bonded to the core. Substituents coupled to the carbon atoms have an electronegativity of less than about 4.

This Application is a Divisional of prior application Ser. No.10/802,973 filed on Mar. 17, 2004, currently pending, to Zhenan Bao, etal. The above-listed Application is commonly assigned with the presentinvention and is incorporated herein by reference as if reproducedherein in its entirety under Rule 1.53(b).

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of AdvancedTechnology Program Cooperative Agreement No. 70NANB2H3032 awarded by theNational Institute of Standards and Technology.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to a method ofmanufacturing organic field effect transistors (OFETs).

BACKGROUND OF THE INVENTION

There is growing interest in the use of OFETs, where an active channelof the transistor is made from an organic semiconductive material. Theorganic semiconductive material should be capable of supporting, under abroad range of environmental conditions, a channel of holes or electronswhen the device is switched on. Ideally, the mobility of carriers in thechannel is resistant to the environmental effects of elevated humidity.

Unfortunately, the performance of conventional p-type organicsemiconductive materials is highly sensitive to moisture. Consequently,the mobility of holes in conventional p-type OFETs rapidly decreaseswith increased humidity. Previous attempts to reduce moisturesensitivity includes placing an encapsulating barrier layer over thesemiconductor molecules. The use of a barrier layer, however, increasesthe expense and complexity to fabricate the OFET. Moreover, a barrierlayer may not be desirable in applications where the organicsemiconductor material serves as a sensor for the detection of targetorganic species in air or aqueous media.

The present invention overcomes the disadvantages associated with priorart devices by providing an OFET having a p-type channel composed offluorinated organic semiconductor molecules with reduced sensitivitytowards moisture, and a method for the fabrication of such a device.

SUMMARY OF THE INVENTION

The present invention benefits from the realization that the moisturesensitivity of OFETs can be reduced through the use of a semiconductorlayer having fluorinated organic semiconductor molecules. The moleculesare configured such that the mobility of holes through the organicsemiconductor molecules is unaffected by the presence of the fluorineatoms. Hole mobility is preserved by including spacer groups between thefluorine atoms and the core of conjugated Pi bonds of the organicsemiconductor molecules. This, in turn, allows the manufacture of OFETshaving higher moisture resistance.

One embodiment of the present invention provides an OFET. The OFET isconfigured to function as a p-type OFET. The OFET comprises a substratehaving a top surface and a semiconductor layer located over the topsurface. The semiconductor layer comprises organic semiconductormolecules. Each of the organic semiconductor molecules includes a corehaving conjugated pi bonds, a fluorinated alkyl group, and an alkylspacer group having a chain of two or more carbon atoms. One end of thechain is bonded to the fluorinated alkyl group and another end of thechain is bonded to the core. Substituents coupled to the carbon atomshave an electronegativity of less than about 4.

In another embodiment, the invention further provides a method ofmanufacturing an OFET configured to function as a p-type semiconductor.The method includes providing a substrate, forming a gate over thesubstrate. A semiconductor layer, comprising the above-described organicsemiconductor molecules, is formed over the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detaileddescription, when read with the accompanying FIGUREs. Various featuresmay not be drawn to scale and may be arbitrarily increased or reducedfor clarity of discussion. Reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 illustrates a detailed sectional view of a top contact organicfield effect transistor embodying the principles of the presentinvention;

FIG. 2 illustrates detailed sectional view of a bottom contact organicfield effect transistor embodying the principles of the presentinvention;

FIG. 3 illustrates detailed sectional view of a alterative organic fieldeffect transistor embodying the principles of the present invention; and

FIGS. 4A to 4F schematically illustrate detailed sectional views of amethod of manufacturing a organic field effect transistor at selectedstages of manufacture.

DETAILED DESCRIPTION

The present invention recognizes for the first time, the advantageoususe of fluorinated organic semiconductor molecules to provide amoisture-resistance p-type channel in an OFET. This recognitionbenefited from an extensive investigation of the moisture sensitivity ofOFETs having channels composed of a broad range of different p-type andn-type semiconductor materials. It has been found that the moistureresistance of certain channels composed of n-type organic semiconductormolecules is due to the presence of fluorine substituents in thesemolecules.

The use of fluorine-substituted organic semiconductor molecules in ap-type channel is counterintuitive because fluorine substitutiontypically causes an otherwise p-type organic semiconductor molecule tobecome an n-type semiconductor. It is thought that the electronwithdrawing properties of fluorine lower the energy levels of the lowestunoccupied molecular orbital (LUMO) and highest occupied molecularorbital (HOMO) of the conjugated system of pi bond in the semiconductormolecule. This, in turn, makes it easier for the LUMO level to acceptelectrons and therefore become a majority electron carrier instead of amajority hole carrier. Therefore one would expect the beneficialmoisture resistance effect conferred by fluorine substitution of anorganic semiconductor molecule to be offset by the deleterious effect ofthe molecule losing p-type characteristics.

The present invention resolves this obstacle through the design of anovel fluorine-substituted p-type organic semiconductor molecule. Aspacer group in the molecule separates a fluorinated alkyl group from acore comprising the conjugated system of pi bonds. The spacer group iscomposed of an alkyl chain having two or more carbon atoms withoutsubstituents having a high electronegativity being directly bonded tothe carbon atoms. While not limiting the scope of the invention, it isbelieved that the location of the alkyl chain of the spacer groupbetween the fluorinated alkyl group and the core preserves the p-typecharacteristics of the organic semiconductor molecule. That is, thespacer group dampens the electron withdrawing inductive effect that thefluorine atoms would otherwise have on the core. The fluorinated alkylgroups, however, still impart moisture resistance to a channel composedof the organic semiconductor molecules.

FIG. 1 illustrates a cross sectional view of a portion of an exemplaryOFET 100 that embodies the principles of the present invention. The OFET100, configured to function as a p-type semiconductor, can be used inany number of applications, such as biosensors, integrated circuits,displays and memory devices.

The OFET 100 includes a substrate 105 having a top surface 110. Thesubstrate 105 preferably comprises a plastic, although otherconventional materials, such as silicon can be used. Examples ofsuitable plastics include polyester, polyimide, or polyamide.

The OFET 100 further includes a semiconductor layer 115 on the topsurface 110 of the substrate 105. The semiconductor layer 115 comprisesorganic semiconductor molecules. Each organic semiconductor moleculeshas a core having conjugated pi bonds and at least one fluorinated alkylgroup. The organic semiconductor molecules also have at least one analkyl spacer group, having a chain of two or more carbon atoms. One endof the chain is bonded to the fluorinated alkyl group and another end ofthe chain is bonded to the core.

The alkyl spacer group can have substituents coupled to the carbon atomsof the chain, provided those substituents have an electronegativity ofless than about 4. Electronegativity is as defined in THE NATURE OF THECHEMICAL BOND, Linus Pauling, incorporated by reference herein in itsentirety. Electronegativity values are expressed here using the Paulingscale, although other analogous scales could be used. One of ordinaryskill in the art would understand how to determine or calculate theelectronegativity value for both atoms and organic substituent groups.

Substituents having the above-described electronegativity, do notsubstantially decrease the mobility of the majority carrier, e.g.,holes, through the core. The absence of strongly electron-withdrawingsubstituents on the carbon atoms ensures that there are no substantialelectron-withdrawing inductive effects on the core. An example of anunsuitable substituent is fluorine, which has an electronegativity ofabout 4. In some cases however, it is preferable to use substituentshaving an electronegativity of less than about 3.5, because such groupsmay still have enough electron withdrawing ability to allow electroninjection into the semiconductor. Examples of unsuitable substituents inthis category include cyano (—CN) and oxygen (forming —C═O with carbon).In still other cases, to further avoid decreases in carrier mobility, itis preferable to use substituents having an electronegativity of lessthan about 3. Examples of unsuitable substituents in this categoryinclude chlorine.

One class of suitable fluorinated alkyl groups are linear alkyl groups,as represented by the formula: C_(n)H_(2n+1−m)F_(m). The number ofcarbon atoms, n, is preferably between 1 and 18. The number of fluorineatoms bonded to the carbon atoms, m, is between 1 and 2n+1 (e.g., alkylchain ranging from monofluorinated, when m=1, to perfluorinated, whenm=2n+1). Examples of preferred fluorinated linear alkyl groups includeperfluoroheptyl (C₇F₁₅) and perfluoropropyl (C₃F₇). Alternatively, otherclasses fluorinated alkyl groups, such as branched alkyls or cycloalkylscan be used.

As noted above, the spacer group is an alkyl group having a chain of twoor more carbon atoms that prevents the fluorinated alkyl group fromhaving a detrimental electron-withdrawing inductive effect on the core.Preferably, the number of carbon atoms in the spacer group is between 2and 18. Any of the above-described classes of linear, branched orcylcoalkyl groups are also suitable for use as a spacer group as well,provided that the alkyl group does not contain substituents havingsubstantial electron-withdrawing properties, as discussed above. Forexample, in some preferred embodiments, the spacer group isnon-fluorinated, or non-halogenated.

The core has a conjugated pi system, as exemplified by oligothiophenessuch as alpha-sexithiophene. Preferably, the organic semiconductormolecules of the semiconductor layer 115 has an ordered crystalline orpolycrystalline structure. For the purposes of the present invention, anoligothiophene has between 2 and 100 repeating units of thiophene. Othercompounds, however, are also within the scope of the present invention.Nonlimiting examples include oligophenyl compounds or combinations ofdifferent benzoid aromatic ring structures like benzene, napthalene oranthracene rings coupled to each other in a conjugated structure.Additional examples include pentacene, nonbenzoid aromatic rings,heterocylic rings, such as thiophene, or co-oligomers of thesestructures, such as co-oligo(bithiophenefluorene),co-oligo(bithiopheneanthracene). Another example is phthalocyanine.

Some preferred organic semiconductor molecules of the semiconductorlayer 115 have a core with substantially coplanar aromatic groups, asthis facilitates a denser and more uniform packing of the molecules. Adense and uniform packing, in turn, improves the carrier mobility of thechannel in the semiconductor layer 115. The term substantially coplanararomatic groups refers to at least two adjacent aromatic groups in anorganic semiconductor molecule having a twist angle of less than about23 degrees between aromatic groups. For example, adjacent thiophenerings in alpha-sexithiophene have a twist angle of about 10 degrees, andtherefore the thiophene rings are considered to be substantiallycoplanar. In contrast, the two linked benzene rings in a biphenylstructure have a twist angle of about 23 degrees, and therefore thebenzene rings are not coplanar.

In some preferred embodiments, the organic semiconducting molecules ofthe semiconductor layer 115 have a core comprising linear organic groupsbecause this facilitates denser and more uniform packing of the organicsemiconductor molecules. Examples of linear organic groups includeoligothiophenes such as alpha-sexithiophene, as shown below:

Other examples of linear organic groups include derivatives ofthiophenes, such as co-oligo(bithiophenefluorene) such as shown below:

In other embodiments, the organic groups of the core can havesidechains, such as oligothiophenes with an alkyl chain substituted atthe 3-positions of one or more thiophenes, such as shown below:

In some embodiments, it is advantageous for the organic semiconductingmolecule to have at least two of non-fluorinated alkyl spacer groupsconnected to ends of the core. In such configurations, one end of thechain of the spacer group is connected to the fluorinated alkyl groupand the other end of the chain is connected to the core. Examples ofsuch organic semiconducting molecules can be represented by the formulaas shown below:

CR corresponds to the core, FAG1 and FAG2 correspond to first and secondfluorinated alkyl groups, respectively, and SG1 and SG2 correspond tofirst and second spacer groups, respectively. FAG1 can be the same ordifferent from FAG2, as can SG1 be the same or different from SG2.Examples of suitable organic semiconductor molecules include5,5′′′′′′-bis(4,4,5,5,6,6,7,7,8,8,8undecafluoro-octyl)alpha-sexithiophene and 5,5′′′′′′-bis(4,4,5,5,6,6,6heptafluoro-hexyl)alpha-sexithiophene.

More than two fluorinated alkyl group-spacer group pairs can beconnected to the core. Using abbreviations analogous to that used aboveto represent the core, fluorinated alkyl groups, and spacer groups, anexample of such an organic semiconducting molecule is shown below:

Additionally, the fluorinated alkyl group can be connected to aninternal carbon atom of the spacer group, but not the carbon atom thatis bonded directly to the core, such as represented below:

Similarly, one or more of the spacer groups and fluorinated alkyl groupcan be connected to an inner aromatic ring of the core, such asrepresented below:

Preferred locations of fluoro-alkyl substitution relative to the coredepend on the size and orientation of the organic semiconductormolecules. For instance, it is preferable for the organic semiconductormolecules to be configured such that a substantial number of thefluoro-alkyl groups between the core and the environment surrounding thesemiconductor layer 115. For shorter oligomers, having between about 2and about 10 repeating units, fluoro-alkyl substituents preferably areon the first or last thiophene ring. Terminal substitutions of suchgroups are preferred because the longer axis of the core in shortoligomers tends to be normal to the substrate surface 115. Thus, thefluorine atoms at one end of shorter oligomers will be at or near thetop surface of the semiconductor layer 117. Additionally, in suchembodiments, fluoro-alkyl substituted organic semiconductor moleculescan advantageously prevent deleterious moisture penetration throughwater permeable substrate 105 and into the core of the organicsemiconductor molecules.

In contrast, the longer axis of the core for long oligmers tend to layparallel to the substrate surface 115. Therefore, better moistureprotection is afforded by fluoro-alkyl substituents located on each, orevery other, aromatic ring of the core. For example, for longeroligomers, having greater than about 10 repeating units, a preferredsubstitution for the fluoro-alkyl group is at the 3-position of thethiophene ring, with every, or every other thiophene ring beingsubstituted.

The use of a low molecular weight organic semiconductor molecule isconducive to the advantageous use of vacuum deposition procedures forforming the semiconductor layer 115, as further discussed below. Somepreferred organic semiconductor molecules have a molecular weight of2000 grams/mole or less. To keep the molecular weight of the organicsemiconductor molecules low, the core preferably has ten or lessaromatic rings.

The semiconductor layer 115 supports a channel region 120 when the OFET100 is active. One skilled in the art would understand that generally,the channel region 120 is located in semiconductor layer 115 near thegate structure 140. The thickness 122 of the semiconductor layer 115 canbe varied to provide channel regions 120 of different sizes. Forinstance, the semiconductor layer 115 can have a single layer of organicsemiconductor molecules or multiple layers. Consequently, thesemiconductor layer's thickness 122 can be about 10 Angstroms or more.In some preferred embodiments, the semiconductor layer 115 has athickness 122 between about 50 and about 1000 Angstroms. In certainembodiments, a thickness 122 of less than about 500 Angstroms ispreferred because the carriers are confined mostly in a channel region120 that is within 50 Angstroms of the surface 122 of the semiconductorlayer 115 nearest to the gate structure 140.

Improved moisture resistance, through the use of fluorinated organicsemiconductor molecules in the semiconductor layer 115, result in thechannel region 120 having higher conductivity in humid enviroments. Asan example, the channel region 120 can have a conductivity in anenvironment having a relative humidity of about 80 percent that is atleast about 70 percent of the conductivity of the semiconductor layer115 in a substantially zero-humidity environment.

The OFET 100 can further include source and drain electrodes 128, 130 onthe top surface 110, with a portion 132 of the semiconductor layer 115being between the source and drain electrode 128, 130. In preferredembodiments, the source and drain electrodes 128, 130 are made of metal,such as silver, platinum or gold, or of conductive organic polymer. Agap 135 between the source and drain electrodes 128, 130 can range frombetween about 1 and about 400 microns. In some OFETs 100, a large gap135 of between about 200 and 300 microns is used, while in other OFETs100, a small gap 135 of between about 4 and about 12 microns is used.OFETs having a small gap, made via photolithography, advantageously havehigher switching speeds and current output. Alternatively, OFETs havinga large gap can be more easily made than small-gap OFET by using shadowmask or print technology.

The substrate 105 of the OFET 100 can further include a gate structure140 that includes a gate electrode 145 and a gate insulator 150interposed between the gate electrode 145 and the source and drainelectrodes 128, 130. The gate electrode 145 can be made of similarmaterials used for the source and drain electrodes, 128, 130, such asgold. The gate insulator 150 can be composed of any number of dielectricmaterials including silicon oxide or aluminum oxide.

FIG. 1 illustrates a particular configuration of the OFET 100 that isknown by those skilled in the art as a top contact OFET. Such aconfiguration is desirable because it facilitates the formation ofelectrical contacts with the source and drain electrodes 128, 130. Insuch a configuration, at least a portion 155 of the semiconductor layer115 is interposed between the substrate 105 and the source and drainelectrodes 128, 130.

Turning now to FIG. 2, a bottom contact OFET 200 is illustrated. Thesame reference number as used in FIG. 1 are used to illustrate similarstructures. A bottom contact OFET 200 configuration is advantageous whena small gap 135 between the source and drain electrodes 128, 130 isdesired. This follows because a wider variety of methods can be used toformed the source and drain electrodes 128, 130, without the concern ofdegrading the organic semiconductor molecules of the semiconductor layer115. There is a decreased risk of degradation because the organicsemiconductor molecules of the semiconductor layer 115 are depositedafter the source and drain electrodes 128, 130 are formed. In such aconfiguration, at least a portion 205 of the semiconductor layer 115 isinterposed between the source and drain electrodes 128, 130. In someembodiments, another portion 210 of the semiconductor layer 115 coversthe source and drain electrodes 128, 130.

Turning now to FIG. 3, another configuration for the OFET device 300 ofthe present invention is illustrated. The same reference numbers as usedin FIG. 1 are used to illustrate similar structures. Here, thesemiconductor layer 115 is on the top surface 110 of the substrate 105with the source and drain electrodes 128, 130 on the top surface of the117 semiconductor layer 115. The gate insulating layer 150 and gateelectrode 145 are over the semiconductor layer 115 and between thesource and drain electrodes 128, 130.

FIGS. 4A to 4F illustrate selected steps in an exemplary method of thepresent invention to form an OFET device 400. The OFET 400 is configuredto function as a p-type semiconductor. Turning first to FIG. 4A, themethod includes providing a substrate 405 comprising, for example,polyimide or other materials as described above. FIG. 4B shows the OFET400 after forming a gate structure 410, comprising a gate electrode 415and a gate insulator 420 over the substrate 405.

The gate electrode 415 can comprise metals such as gold, silver,platinum, and palladium. These materials can be deposited byconventional techniques like vacuum deposition, thermal evaporation orelectron beam evaporation, followed by patterning via conventionallithographic processes to define the gate's structure. Alternatively,the gate electrode 415 can be made of conducting polymers, such aspolyaniline or polythiophene, that can be doped to increaseconductivity, or conductive ink, comprised of graphite and conductingpolymers. Conventional techniques, such as ink jet printing, screenprinting, or molding, can be used to form the gate electrode 415.

The gate insulator 420 can be formed by depositing an inorganic layer,such as silicon oxide or aluminum oxide sputtered over the substrate405, and preferably on the gate electrode 415. Alternatively, the gateinsulator 420 can be formed by spin-coating insulating organic polymersor organic polymer/inorganic composites, or by chemical vapor depositionof monomer or organic polymers, such as poly-para-xylylenes likeparylene, on the gate electrode 415. As an example, polymers such aspolyimide or polymethylmethacrylate can be deposited alone or incombination with titanium nanoparticles that serve to increase thedielectric constant of the gate insulator 420. See e.g., U.S. patentapplication Ser. No. 10/700,651, by Howard E. Katz et al., filed Nov. 4,2003, incorporated herein in its entirety.

There are a number of processes by which a semiconductor layer of thepresent invention can be formed over the substrate 405. For instance,the manufacture of a top contact OFET 400 can include steps illustratedin FIGS. 4C and 4D. As illustrated in FIG. 4C, the semiconductor layer425 is formed over the substrate 405, and preferably on the gateinsulator 420. Then, as shown in FIG. 4D, source and drain electrodes430, 435 are formed over the gate 410 and on the semiconductor layer425.

Preferably the semiconductor layer 425 is formed between the source anddrain electrodes 430, 435. In such OFETs 400, at least a portion 440 ofthe semiconductor layer 425 can be interposed between the source anddrain electrodes 430, 435 and the substrate 405. Of course, the sourceand drain electrodes 430, 435 can be comprised of the same or differentmaterials, and formed using the similar or different processes asdescribed above for the gate electrode 415.

Alternatively, the sequence illustrated in FIGS. 4E and 4F may be usedto manufacture a bottom contact OFET 400. Source and drain electrodes430, 435 are formed over the substrate 405, and preferably on the gateinsulator 420, of the partially completed device in FIG. 4B, to yieldthe OFET structure 400 in FIG. 4E. Then, a semiconductor layer 425 isformed over the substrate 405 to produce the OFET structure 400 shown inFIG. 4F. In such OFETs 400, at least a portion 445 of the semiconductorlayer 425 is interposed between the source electrode 430 and the drainelectrodes 435. In some embodiments, as shown in FIG. 4F, thesemiconductor layer 425 covers the source and drain electrodes 430, 435.

The semiconductor layer 425 can be formed using any number ofconventional techniques. For instance, when the organic semiconductingmolecules of the semiconductor layer 425 have a molecular weight of 2000gram/mole or less, vacuum sublimation can be used. Vacuum sublimation isadvantageous because this promotes the formation of a uniformcrystalline thin film of semiconducting molecules with a high degree ofreproducibility. Of course, other procedures such as spin-coating ordip-coating can be used to form the semiconductor layer 425.

Although the present invention has been described in detail, those ofordinary skill in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thescope of the invention.

1. A method of manufacturing an organic field effect transistor (OFET),comprising: providing a substrate; forming a gate over said substrate;and forming a semiconductor layer over said substrate, wherein saidsemiconductor layer comprises organic semiconductor molecules, whereineach of said organic semiconductor molecules includes a core havingconjugated pi bonds, a fluorinated alkyl group, and an alkyl spacergroup having a chain of two or more carbon atoms, wherein one end ofsaid chain is bonded to said fluorinated alkyl group and another end ofsaid chain is bonded to said core and substituents coupled to saidcarbon atoms have an electronegativity of less than about 4; and whereinsaid OFET is configured to function as a p-type OFET.
 2. The method ofclaim 1, further including forming source and drain electrodes over saidgate, wherein said semiconductor layer is between said source and drainand at least a portion of said semiconductor layer is interposed betweensaid source and drain electrodes and said substrate.
 3. The method ofclaim 1, wherein forming said gate further includes forming a gateelectrode on said substrate and forming a gate insulator on said gateelectrode.
 4. The method of claim 1, wherein forming said semiconductorlayer includes vacuum sublimation of said organic semiconductormolecules on said substrate.
 5. The method of claim 1, wherein formingsaid semiconductor layer includes spin-coating or dip-coating saidorganic semiconductor molecules on said substrate.
 6. The method ofclaim 1, wherein said semiconductor layer is formed such that at least aportion of said channel is interposed between a source electrode and adrain electrode.
 7. The method of claim 6, wherein said channel isformed such that said semiconductor layer covers said source and drainelectrodes.